1. Field of the Invention
The present invention relates to radio communication systems, and more particularly, to an RF transceiver having a digital frequency synthesizer in which certain parameters relating to the time for the frequency synthesizer to lock onto a desired frequency are monitored in order to provide a measure of performance of the transceiver.
2. Description of Related Art
Spread spectrum modulation techniques are increasingly desirable for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, a radio frequency (RF) carrier is shifted in discrete increments in a pattern dictated by a predetermined sequence. These spread spectrum systems are known as "frequency hopping" modulation systems, since the transmitter jumps from frequency to frequency in accordance with the predetermined sequence. Another type of spread spectrum communication system utilizes an RF carrier modulated by a digital code sequence having a bit rate, or chipping rate, much higher than the clock rate of the information signal. These spread spectrum systems are known as "direct sequence" modulation systems. The RF carrier may be modulated such that a data stream has one phase when a code sequence represents a data "one" and 180.degree. phase shift when the code sequence represents a data "zero."
In either type of spread spectrum system, a digital frequency synthesizer is typically used within a phase-locked loop circuit to generate the RF carrier. The frequency synthesizer may be controlled by a microprocessor, which provides control signals to determine the frequency on which the frequency synthesizer operates. Frequency hopping systems require frequent changes in frequency, and the microprocessor determines the pattern and timing of the frequency changes. Rapid frequency changes may also be necessary in direct sequence systems as well as in other types of RF transceivers, such as to switch between transmitting and receiving modes. A drawback of such frequency synthesizer circuits is that there is an inherent delay period during which the frequency synthesizer stabilizes, or locks onto, a new frequency. This delay period is referred to as the locking time. The locking time of a frequency synthesizer is an important measure of the performance of a radio system, and certain radio system protocols specify stringent locking time parameters.
Radio system manufacturers typically measure the frequency synthesizer locking time using external equipment coupled to the transceiver while it is disassembled to expose the frequency synthesizer circuitry. After the transceiver has been delivered to a customer, however, it is impractical to perform such measurements without returning the transceiver to the manufacturer. As a result, reductions of the frequency synthesizer performance are often not recognized until the radio system has degraded below acceptable levels. Moreover, the factory tests of the frequency synthesizers do not accurately reflect real operating conditions of the radio system in which system power demands and noise affect the operation of the frequency synthesizer, and thus do not provide a reliable indicator of actual performance.
Accordingly, it would be desirable to provide a radio system which constantly monitors the operation of the frequency synthesizer to detect subtle changes in performance, particularly with respect to the locking time of the frequency synthesizer. It would further be desirable to provide a radio system in which the operating dynamics of the frequency synthesizer can be evaluated under real operating conditions after delivery to a customer.